This invention relates to high pressure multi-stage centrifugal pumps used extensively though not exclusively for handling large volumes of often hazardous fluids in oil wells and the like where it is critical for safety, health, environmental and other reasons to minimise the risk of leakage. More particularly, the invention relates to means for preventing rotation of an inter-stage bushing or ring in a high pressure multi-stage centrifugal pump with an axially split casing which houses a shaft carrying impellers.